Cutting an opening in unreinforced CMU wall 2

[asixth]

Hi guys,

I have the situation that has come up where a client wants to provide a new 9'(2700) wide doorway through and un-reinforced block. My job is to strengthen this wall so a lintel can span across the opening.

My initial thoughts were to strengthen the opening with steel and then rely solely on the steel to span the opening.

Has anyone had problems cutting unreinforced masonry to provide a doorway and what is the best detail I can use for this application?

 

[ishvaaag]

It seems not to have significant loads. One might overengineer these things. You may drill for some bars supported on props temporarily discharge the loads, or not even this, work 1st one side and use steel L and then the other, or make an incision from one side and insert rebar, there are many solutions from the more risky to the absolutely safe, that would be like for an structural bearing wall cut.

 

[rowingengineer]

I have had problems in the past with workmen whom over cut the corners instead of drilling a large hole and cutting from the hole, thus that crack don’t continue.

The crack continued for only a few mm before i had them drill a hole and stop the crack. However they did have to patch the job. I also have this problem when they cut holes in tilt panels. I now have a heavy spec in this regard.

I personally don't have any problems with the steel idea, there are many ways to do this, I personally like to over cut the door in height, install lintel blocks & reo and reinforce the existing masonry, seal it all up with concrete, looks good, and architects likes it.


When in doubt, just take the next small step.

 

[vandede427]

I use the following detail a lot with success.

1. rake out mortar joints on either side of wall at the elevation of the top of the doorway/window.
2. slide in an angle on either side. make sure length of angle is 16" longer than opening for 8" bearing each side.
3. remove wall below.
4. weld bottom closure plate to horizontal leg of angles. this keeps debris in the wall and makes all three steel shapes into one composite shape.

 

[jechols]

I have also through-bolted a channel each side, then remove the wall below, and then welded the bottom plate to the channels.

 

[spats]

You have a lot more to deal with than just creating the header. The block on the sides of the opening has to be designed to carry not only all loads above the opening, but the lateral wind loads on the door as well.

 

[KootK]

RowingEngineer,

I typically do the steel angle thing too. Your method sounds great though -- especially if architects dig it. Can you list the construction steps in more detail though? I'm not entirely sure that I understand. Also, do you need to shore the opening using your method?

One more too: are you proposing that reinforcement be added to the existing masonry wall above the opening? If so, I wasn't aware that was possible / practical. If it is that's great news. I'll have a lot more options available to me when renovating masonry.

Thanks.

 

[MaddEngineer]

You can call for saw cutting of the opening.

However, a couple of additional comments:

1. Check if the lintel assembly will have to be fire rated, e.g., 1HR, 2HR, 3HR, etc. This may require the istallation of fire-proofing plates and use of a W8 or larger beam.

2. 9'-0" is an odd number when dealing with CMU. Try to keep to 8" modules. So the rough opening (R.O.) would be either 9'-4" or 8'-8". This will ensure that you will have a cleaner installation. Same goes for the head height.

3. Will a finished door be installed in the opening or will it be a pass-thru opening. If something else will be installed inside the opening make sure you take account of build-out clearances.

4. You may have to specify that the bearing cells be grouted solid. Hollow CMU is typically good for 45psi on the gross area based on empirical design methods.

5. You seem to be concerned with construction means and methods, some will leave it up to the contractor to provide shoring and sequence design by his/her engineer. What is common is the use of needle beams or temporary channels bolted to both sides of the wall to span the temporary opening. Remember the temporay opening will be 16" larger than the permanent opening in order to set the bearing points of the lintel/header.

 

[asixth]

Thanks for the input guys, I am heading out there this morning so I will tell you how it all goes.

 

[rowingengineer]

Kootenaykid,
Yes you need to shore up the existing masonry when you use this detail, And yes this is tricky. Generally I sort this out with the builder, as he will have some great ideas, but the method that I specify on the drawings, is fairly similar to Vandede427, but a touch different
general spec.
1. Drill out edges of opening, using a 50mm bit.
2. cut out top of opening, install angle each side of opening (I like 150x75x5 UA) Weld some temp props in place, how many props decides the angle size.
3. Finish cutting out wall and install lintel blocks (horizontal reo at the same time This must be Doweled back into existing masonry 300 odd mm). middle lintel block to have large hole installed, about 100mm will do the job (if a longer lintel is require you will need a few more holes and back prop under lintel (can now remove the 150x50x5 UA)
4. Depending on existing masonry, if hollow; reinforce with reo, by drilling a large hole for feeding in reo and concrete, if corfilled check strength of existing if ok, do nothing is not I like to use a bit of carbon fibre (cost $$$, but again if an architect is involved it will be worth it).
5. using a fast flow concrete mix (generally 10mm agg) check with concrete supplier, fill all. There are some special techniques involved to make sure that the lintel is fully core filled, an experience concreter will know them, but generally a bit of flexible pipe will do the job
6. install new door.

Note, Make sure the side of the wall you put the holes is either going to be rendered or covered. Otherwise, you will need to use masonry/block reflection formwork ($$$).

As I said this is like by architects but one problem is that it is $$$$. I have been doing a lot of reno's for million dollar houses so it wasn't a great problem, even when i asked for carbon fibre.




When in doubt, just take the next small step.

 

[KootK]

Thanks RowingEngineer. When I use your detail, I'll remember to pay homage in my specs to "Random Interet Guy".

I've got a couple more -- possibly naive -- questions:

1) When you drill out the edges of the opening, this means small, evenly spaced holes tracing out the opening, right? I take it the intent is to be able to make the cuts without initiating some undesirable cracking? What's a good spacing to use?

2) When you remove the steel angles, how do you fill in the vertical space between the new lintel and the existing CMU that would be left behind?

3) Is it only the lintel dowels carrying the shear into the wall or do you extend the lintel block itself a couple of cores into the wall?

Again, thanks for sharing -- great stuff.

 

[rowingengineer]

Kootenaykid,
You could put what ever you like in your spec, I sure noone reads it.
1. Genrally the Holes are only in the cornes of the cuts so that the saw cuts (or other) has a good starting place.
2. The removing of the angle is a fun one, Genrally this will depend on the opening size. I normally get them to remove the angle a little bit a a time using a 10mm packer as another temp support geranlly at about 300-400 centres, (they don't need to be that big) grout up inbetween and over the packers, make sure there back form the edge if your the builder. (should have put this in my orginal post, very important). But in saying that, last week they just use a sauge long set gout for the linte, didn't embed teh angle to far from the edge (about 15mm hang and then just grout over the top of the sauge t teh end.
3. This all depend on your loading, If heavly loaded i just prop the roof and completly replace hte lintel, if not, just useing the dowel action.


When in doubt, just take the next small step.

 

[rowingengineer]

Here is a quick sketch to see what i am saying, because even i go lost in trying to read my last post.

When in doubt, just take the next small step.

 

[asixth]

Thanks for your help guys, I have a photo. Basically, there is an existing 900 wide doorway that you can see at closest of the image, the new 1200 wide door is before the returns around the corner. The wall is reinforced and core filled every second core, I need to remove the wall while providing a lintel that can span the 2700 while supporting the skylight above.

 

[StructuralEd]

We try not to tell the contractors the means and methods of performing work. That would transfer liability for the physical installation to the designer (you), and you are only supposed to be responsible for design.

Design the lintel and leave the construction techniques to the contractor.

 

[KootK]

RowingEngineer,

Thanks for taking the time to make the sketch. It has caused me to have a couple of additional questions though:

1) What purpose do the blockouts serve? Aren't you concerned with the eccentricity of the load on the angles with that little embedment depth? Do you count on the props to absorb that load somehow?

2) What on earth is a "grout sausage"? Is it tasty?

I agree with SED's concerns regarding liability. However, for something like this, there's really no way to provide an economical / practical solution without giving some consideration to the construction process involved. Whether or not you show it on your drawings is another matter I suppose.

 

[rowingengineer]

asixth,
Do you have an existing bond beam? (Give the wall is reinforced every second core i would say that there is a good chance of a 3 course bond beam, that would span teh distance). Do you have the original engineering? Is there a lintel over the door reinforced? I assume this isn't a bracing wall?

My thoughts would be, is the bond beam strong enough? If not it looks that you have a ceiling behind the door at about door level? Hence your design doesn’t have to look good on that side. Given it is reinforced every second core you could put a 200-300 PFC against the wall and chemical anchor into the wall, at each reinforced core.

Kootenaykid,
1. Given the design I have been talking about and what asixth has are completely different, my design is for a unreinforced wall (old residential style in my area), hence the loads are generally very small, and the angle can handle the flange bending (I have had to upsize the angle in thickness before to account for the flange bending) and minimum embedment normally isn't a problem (the condition of the blocks will govern this however, bad blocks means further embedment, and then I do a formed lintel. The purpose of the block outs is so that the props are about 250mm apart and outside the working area so you can get your new lintel in.

2. grout sausage is yummy especially if you normally have a table spoon of cement before excise. A grout sausage is where you place grout inside a plastic package (normally these are used in tilt panel construction for times when you want to land a panel on a strip footing and have full bearing). Depending on the grout used will decide in what time these set, but given the packaging effect you can be assured of a good bearing. Normally these grout sausages can come in different sizes, length and dia.


When in doubt, just take the next small step.

 

[asixth]

rowingengineer,

They where my initial thoughts as well, to chemical anchor a channel to the blockwall at each location where there is core-filling (16" for every second course). This option will also make things easier because we don't need to remove anything before we can install the channel and start removing the block wall.

A senior in our office thought we should try and get the wall to bear on the channel opposed to having the load go through anchors before being supported by the channel. Just as a more direct load path.

 

[KootK]

You could do this:

1) install channels with anchors only
2) remove block
3) install a bottom plate connecting the channels with grout sausages between the plate & cmu.

There would be a temp condition w/o the direct load transfer buy you'd have it in the finished product. Rowing: let me know if I've misused the sausages here.

I've used an identical detail for a load bearing conc wall, minus the sausages.

Alternately: is there a suitable adhesive that could be used between the block and steel?

 

[rowingengineer]

asixth,
Due to the stiffness of the wall/header (6 blocks deep I assume), the load path you provide will only be effective in serviceable state if the beam is stiffer then the header/lintel. I would say the header is going to be very stiff due to arching action alone, hence your beam would need to be something ridiculous. my take is normally to check and see what the existing Bond beam can do, if it can take the service loads, then i only provide a beam for ultimate loads, let the header crack after service load. However if lintel/header has nothing of value my thoughts would be install a chemically anchored beam PFC as above, and tension it up, using a joint in the centre, and TB bolts, a gap of about 10-20mm should do the job. This ensures the header is using the beam as tension reinforcement. Ie a big strut tie beam.

KootenayKid,
Yep that detail would be used in your example. The use of an adhesive here like would be appropriate, never have used it myself, but i can see nor reason it wouldn’t work if correctly detailed, ie, you would want to keep this separate of the angle, this is where the grout sausage is perfect.





When in doubt, just take the next small step.